Investigation into the Aerodynamics of Swashplateless Rotors Using CFD-CSD Analysis

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Abstract

This study obtains a better understanding of the aerodynamics of integrated
trailing edge flap (TEF) based swashplateless rotors. Both two dimensional (2D)
and three dimensional (3D) analysis/simulations are performed to understand the
behavior of TEF airfoils and integrated TEF based swashplateless rotors.
The 2D aerodynamics of TEF airfoils is explored in detail. A semi-empirical
approach is developed for modeling drag for TEF airfoils in steady flows based
on baseline airfoil drag data alone. Extensive 2D CFD simulations are performed
for a wide range of flow conditions in order to better understand various
aspects of the aerodynamics of TEF airfoils. The trends in the airloads (lift,
drag, pitching moment, hinge moment) for TEF airfoils are obtained. Nonlinear
phenomena such as flow separation, shocks and unsteady vortex shedding are
investigated, and the flow conditions and trends associated with them are
studied.
The effect of airfoil properties such as thickness and overhang are studied.
Various approaches are used to model the effect of gaps at the leading edge of
the flap. An approximate ``gap averaging'' technique is developed, which
provides good predictions of steady airloads at almost the same computational
cost as a simulation where the gap is not modeled. Direct modeling of the gap
is done by using a patched mesh in the gap region. To solve problems (such as
poor grid quality/control and poor convergence) that are associated with the
patched mesh simulations, an alternate approach using overlapping meshes is
used. It is seen that for TEF airfoils, the presence of gaps adversely affects
the effectiveness of the flap. The change in airloads is not negligible,
especially at the relatively higher flap deflections associated with
swashplateless TEF rotors.
Finally, uncoupled and coupled computational fluid/structural dynamics
(CFD-CSD) simulations of conventional (baseline) and swashplateless TEF rotors
is performed in hovering flight. The CFD-CSD code is validated against
experiment and good agreement is observed. It is observed that the baseline
UH-60 rotor performs better than the swashplateless UH-60 rotor. For an
untwisted NACA0012 airfoil based rotor, the performance is similar for the
baseline and swashplateless configurations. The effect of gaps on the
performance of swashplateless TEF rotors is also investigated. It is seen that
the presence of chordwise gaps significantly affects the effectiveness of the
TEF to control the rotor. Spanwise gaps also affect the performance of
swashplateless rotors but their effect is not as significant.